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c56e59776f
With CONFIG_LTO_CLANG_THIN enabled, with some of previous version of kernel code base ([1]), I hit the following error: test_ksyms:PASS:kallsyms_fopen 0 nsec test_ksyms:FAIL:ksym_find symbol 'bpf_link_fops' not found #118 ksyms:FAIL The reason is that 'bpf_link_fops' is renamed to bpf_link_fops.llvm.8325593422554671469 Due to cross-file inlining, the static variable 'bpf_link_fops' in syscall.c is used by a function in another file. To avoid potential duplicated names, the llvm added suffix '.llvm.<hash>' ([2]) to 'bpf_link_fops' variable. Such renaming caused a problem in libbpf if 'bpf_link_fops' is used in bpf prog as a ksym but 'bpf_link_fops' does not match any symbol in /proc/kallsyms. To fix this issue, libbpf needs to understand that suffix '.llvm.<hash>' is caused by clang lto kernel and to process such symbols properly. With latest bpf-next code base built with CONFIG_LTO_CLANG_THIN, I cannot reproduce the above failure any more. But such an issue could happen with other symbols or in the future for bpf_link_fops symbol. For example, with my current kernel, I got the following from /proc/kallsyms: ffffffff84782154 d __func__.net_ratelimit.llvm.6135436931166841955 ffffffff85f0a500 d tk_core.llvm.726630847145216431 ffffffff85fdb960 d __fs_reclaim_map.llvm.10487989720912350772 ffffffff864c7300 d fake_dst_ops.llvm.54750082607048300 I could not easily create a selftest to test newly-added libbpf functionality with a static C test since I do not know which symbol is cross-file inlined. But based on my particular kernel, the following test change can run successfully. > diff --git a/tools/testing/selftests/bpf/prog_tests/ksyms.c b/tools/testing/selftests/bpf/prog_tests/ksyms.c > index 6a86d1f07800..904a103f7b1d 100644 > --- a/tools/testing/selftests/bpf/prog_tests/ksyms.c > +++ b/tools/testing/selftests/bpf/prog_tests/ksyms.c > @@ -42,6 +42,7 @@ void test_ksyms(void) > ASSERT_EQ(data->out__bpf_link_fops, link_fops_addr, "bpf_link_fops"); > ASSERT_EQ(data->out__bpf_link_fops1, 0, "bpf_link_fops1"); > ASSERT_EQ(data->out__btf_size, btf_size, "btf_size"); > + ASSERT_NEQ(data->out__fake_dst_ops, 0, "fake_dst_ops"); > ASSERT_EQ(data->out__per_cpu_start, per_cpu_start_addr, "__per_cpu_start"); > > cleanup: > diff --git a/tools/testing/selftests/bpf/progs/test_ksyms.c b/tools/testing/selftests/bpf/progs/test_ksyms.c > index 6c9cbb5a3bdf..fe91eef54b66 100644 > --- a/tools/testing/selftests/bpf/progs/test_ksyms.c > +++ b/tools/testing/selftests/bpf/progs/test_ksyms.c > @@ -9,11 +9,13 @@ __u64 out__bpf_link_fops = -1; > __u64 out__bpf_link_fops1 = -1; > __u64 out__btf_size = -1; > __u64 out__per_cpu_start = -1; > +__u64 out__fake_dst_ops = -1; > > extern const void bpf_link_fops __ksym; > extern const void __start_BTF __ksym; > extern const void __stop_BTF __ksym; > extern const void __per_cpu_start __ksym; > +extern const void fake_dst_ops __ksym; > /* non-existing symbol, weak, default to zero */ > extern const void bpf_link_fops1 __ksym __weak; > > @@ -23,6 +25,7 @@ int handler(const void *ctx) > out__bpf_link_fops = (__u64)&bpf_link_fops; > out__btf_size = (__u64)(&__stop_BTF - &__start_BTF); > out__per_cpu_start = (__u64)&__per_cpu_start; > + out__fake_dst_ops = (__u64)&fake_dst_ops; > > out__bpf_link_fops1 = (__u64)&bpf_link_fops1; This patch fixed the issue in libbpf such that the suffix '.llvm.<hash>' will be ignored during comparison of bpf prog ksym vs. symbols in /proc/kallsyms, this resolved the issue. Currently, only static variables in /proc/kallsyms are checked with '.llvm.<hash>' suffix since in bpf programs function ksyms with '.llvm.<hash>' suffix are most likely kfunc's and unlikely to be cross-file inlined. Note that currently kernel does not support gcc build with lto. [1] https://lore.kernel.org/bpf/20240302165017.1627295-1-yonghong.song@linux.dev/ [2] https://github.com/llvm/llvm-project/blob/release/18.x/llvm/include/llvm/IR/ModuleSummaryIndex.h#L1714-L1719 Signed-off-by: Yonghong Song <yonghong.song@linux.dev> Link: https://lore.kernel.org/r/20240326041458.1198161-1-yonghong.song@linux.dev Signed-off-by: Alexei Starovoitov <ast@kernel.org> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.